If your editors accept writing actual things like: "You have donated ${DonatedAmt} this year", you could let them do that.

Add a display template like this, filename: XhtmlString.cshtml:

@using EPiServer.Core
@model XhtmlString

@{
    if (Model == null) { return; };
}

@{Html.RenderXhtmlString(Model.ReplacePlaceholders());}

Then, something like this:

public static class XhtmlStringExtensions
{
    public static XhtmlString ReplacePlaceholders(this XhtmlString xhtmlString)
    {
        var contextModeResolver = ServiceLocator.Current.GetInstance<IContextModeResolver>();
        if (contextModeResolver.CurrentMode == ContextMode.Edit)
        {
            return xhtmlString;
        }

        var processedText = xhtmlString.ToInternalString().Replace("${DonatedAmt}", "1234");
        return new XhtmlString(processedText);
    }
}